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  1. #1

    Default GM Metric Suspension. What am I feeling

    I am racing a homebuilt IMCA GM Metric Stock car with a name brand rear end. I've had some success winning 2 out of 10 races this year. As all racers, I am looking for a certain feel and hopefully more speed. I started the year with a 150lb LR spring, 200RR and 100 lbs of bite with me in the car. The feel was great on starts and the car felt stable and took off well, but had to really pedal it in the center of the corner, and once the car was back under me, had really good drive again. As the tracks have dried out, I went higher in bite number, up to 140 only to lose feeling of being "on the track". Seemed as though it made the center-off worse and would feel like instant traction, but felt like the lr would "give up" even quicker and the RR just over drove it. I went down in the RR spring rate to a 175 and def helped tighten that center off feeling, but still need to pedal it. I have a modified background and been following the trends of soft LR springs so I gave that a whirl. I used a 125 with same 140lb bite number (6" of preload at installed height). With this spring it felt like the instant traction wasn't there and lazy/sagged on lr on entry. I didn't have to pedal it as much as the 150 and liked the feel there, but it seems like the car doesn't take off straight (I'd describe it as spinny/squirrley) on the starts. Almost like the RR is dominate to start, then LR..like its climbing between each tire. This last weekend I tossed a 200 LR spring in there and stayed with that 140lbs of bite. The initial traction on starts was back, but the mid-corner pedaling is back and felt like traction ran out even quicker than the 150lbs spring. So my question is what am I feeling? The higher preloaded LR should provide more dynamic bite vs. the others, but is it too soft on starts? Old adage that "stiffer spring gets the weight"? Does that still hold true even when preloaded to same installed load as stiffer rate? Is it the static bite number that is causing this to feel "off the track"? Other ideas? 2"offset around the car with a 4" RR. 53% Left and 54% rear. 7 5/8 ride height across the rear per the rear end builder giving top links 13 deg down hill to the chassis at ride height. Consistently 1-3/4 to 2" travel on RR and LR has been spring dependent..I am all for trial and error and I have found a way to win with it now, but need someone to bounce thoughts off of to avoid wasting a night on R&D.Bananahammack00

  2. #2
    Join Date
    Dec 2007
    Posts
    138

    Default

    Where are your springs located? (front, center or rear of housing) where are your lower trailing arms located? (how far away from housing) do you still have load on LR spring when shock tops out, and just curious, what kind of rearend are you using?

    Curt Drake
    IMCA stock car # 55

  3. #3

    Default

    Left Cup is forward as far as we can legally go. RR is center of housing. Lower trailing arm is all the way down from the housing and RR is in the 2nd from top hole. This really means nothing to you as every rearend mfg has different holes. I'd have to get an angle finder out if you're looking for angles. With the 125, yes still some load, the others no. However I'm not traveling it that far with either spring. With the lower ride heights i'm only traveling the LR up to mid-frame rail hump. its a Bigg Johnson Rear end.

  4. #4
    Join Date
    Dec 2007
    Posts
    138

    Default

    I have the same rearend, I also build my own cars...that said what I have is Left forward, Right behind....I use a 150 on LR and a 225 on RR, 120# bite....but at full drop spring still has load on it. When there is traction, leave lowers as is, when slick, raise RR to top hole. What shocks are you running?

    Curt Drake
    IMCA stock car #55

  5. #5
    Join Date
    Dec 2007
    Posts
    138

    Default

    Also, I think your ride height is a little high, I'm 2 1/2" lower on ls, and 1 1/2 lower on rs. Are you using 16" springs?

    Curt Drake
    IMCA stock car #55

  6. #6

    Default

    I tried and thought the 225 and 120# bite made the mid-corner even worse. I also tried adding a 25lb chunk of lead right on top of RR because that tire is quite a bit warmer (feed the heat), but that made it even worse as well. I'd compare it to the RR wanting to drive around the LR. I would have to wait longer until I could pick up the throttle and really feathered the RR. Did your BJ rearend come with the shock mounts and spring cups welded on? Depending on that, we may have different drop outs as mine came in a box. I could give you a C-C at ride height and then a distance from floor to bottom mount. Are you measuring from floor to frame rail for rear heights? 2.5" lower would be under 5.5" to from to ground on LS? I thought the 7 5/8" was low. I have also tried moving the lower trailing arm up on the RR and def gave more bite, but made the car less maneuverable in my opinion. Felt like it really locked it down and wasn't able to drive under someone if I needed to. This loose on throttle is all the time, obviously worse when there is little/no traction. 16" LR 13" RR and I'm on CPD Base Valve integras all around. For your bite # are you with or without shocks? Finding this LR shock adds 15# on the scale at RH even though its BV. Bananahammack00

  7. #7
    Join Date
    Dec 2007
    Posts
    138

    Default

    my ride heights are frame to top of rearend housing....I added my own spring cups and shock mounts.
    If you move right spring cup forward it will take away some of the rr drive. I also have CPD integras, just not base valves, and I unhook lr when scaling....my car steers very well with the front end, and usually only move trailing arm, and take stagger out for dry track. What front springs are you running?

    Curt Drake
    IMCA stock car#55
    Last edited by Drake Racing; 07-14-2021 at 08:48 AM.

  8. #8
    Join Date
    Dec 2007
    Posts
    138

    Default

    Sorry I mean Back on RR cup...its early.

    Curt Drake
    IMCA stock car #55

  9. #9

    Default

    I'm really happy with how it turns other as long as I don't move that trailing arm. I'm at 1.75" stagger for heat races and 1" for features. 1050lf and 950rf springs. I'm on the "JG" lf front shock, R300 RF, CPD "Standard stock car LR" and RRUB. Got to love CPDs naming.. The only changes Ive been making heat to feature is to tie up the LF shock to 1" over ride height, stagger, and every great once and a while maybe a bite change. I'm starting to think I've got my lead too low. When I first built the car, I could only get 51% rear. The car worked well but lacked the straight line drive which I figured was the low rear %. I added ~100lbs of lead under the cell 3/4 back to get 54% rear, but I think its maybe too low and basically swinging the back too much. I think for this weekend I'm going to move that weight up and in front of the fuel cell and see if it stops the swinging. This is the first stock car i've built and we didn't use a name brand to copy so definitely learning on the fly. Its a little harder to get that info when you call up a builder and he knows you don't have one of his cars. -Bananahammack00

  10. #10
    Join Date
    Dec 2007
    Posts
    138

    Default

    Almost the same exact setup as my car....my lead is all even with top of fuel cell...I wouldn't worry about having some towards the back.( I've got 150# behind the rearend centerline) keep your rear % up. (I'm 55.2 at 2975). Try going to
    1/2 or even less stagger when it slicks off, that 1 adjustment really changes my car. You would be surprised at how much front end settings help also. If you add more RF caster the car will tighten up when you backsteer and help to drive off straighter. I think the biggest help will be to move rr spring perch back. Does your car get on rf well, and hike up good?

    Curt Drake
    IMCA stock car 355

  11. #11

    Default

    Yeah It feels decent hike & RF wise. Just looking to get into the fuel faster . I'm at 54% @ 3030lbs so lacking that rear %. I'm going ot start with lead and stagger this weekend. moving the cup gets pretty intensive, but will keep it in mind. Front end is 2 chamber 3.5 caster and rf is 5.75 chamber and 4.5 caster.One question I sort of have regarding rear %. is it really about XX rear percentage? Or more about weight on the wheel? Its really the same thing is it not?
    Last edited by bananahammack00; 07-14-2021 at 11:59 AM.

  12. #12
    Join Date
    Dec 2007
    Posts
    138

    Default

    Let me know how it goes....stagger will surely help tighten the car up...min actually gets to the point where I can be too tight when I pick up throttle. Do you have your shocks on back adjustable for IMCAs 4 1/2" rule? That was something I worked on alot to make sure spring stayed loaded, car got full hike, and I stayed within the 4 1/2" shaft inside shock rule.

    Curt Drake
    IMCA stock car #55x

  13. #13
    Join Date
    Dec 2013
    Posts
    7

    Default

    The stiffer spring gets the weight sooner and gets rid of the weight sooner. If you run the 200 left rear you will have to run more lead weight on the left side of the car. I have had good results running more left rear bite. 200 or more left rear bite. Left side weight is traction. Too much is not good and too little is not good. I would try running a 16 inch 175 with a minimum of 200 lbs bite. 200 13 inch right rear. Have had better luck making these cars go with making front spring changes. I would not be afraid to run an 1100 right front with the 1050 right front. A few things that have worked for me.

  14. #14
    Join Date
    Jul 2007
    Posts
    3,181

    Default

    raise your left rear lower trailing arm all the way up if you want more drive middle off

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